I am aware that there are many HD cheerleaders here roaming the boards and they won't like my view on this. Sorry. But, I think that the proponents of HD have too much mental energy occupied with somehow "saving the AM band" and don't spend enough time using common sense. The technology is specious enough during the more stable daylight hours, but the AM band will not support this technology at night. What will happen (if a good percentage of broadcasters flip on HD exciters) is that we'll have a hash-mash of interference that makes all but the closest and strongest stations unlistenable.
Not to mention that the digital decoder will bounce back and forth between analog and digital with every bit of fade or interference. It will also drop right out for every lightning strike within 250 miles, taking a few seconds to recover the digital encoding.
Now to the common sense part: people aren't killing themselves to buy HD radios. At best, it will be 20 years before they are in the majority (if ever). Until then, you will have a lot of people who wonder why AM sounds so bad.
The most successful AM stations - those 50 kw signals that still do well - will see the edges of their markets erode thanks to adjacent or near-adjacent HD stations. Many of whom still program just as poorly as in plain ol' analog. An example of this is WWDB 860 in Philly - which interferes badly with WCBS starting in the Trenton area. And, what does a business talk/brokered format need HD for anyway? What good does it do? None. All it does is render a population of people unable to listen to a much more popular station from an adjacent market.
I know, I know, if its not in the DMA it gets no consideration and no protection. That concept gives concrete bunker-like cover to HD broadcasters everywhere....but it is not recognized by the listeners who know nothing about the issue. All they know is that they lose one of their NY commute station closer to the city than they used to. And, they will have no more interest in the programming offered on 860 than they have now. WCBS gets a few less listeners on the fringes and FM/IPODs/etc. benefit yet again. Meanwhile, WWDB does not benefit. That's called a 'lose-lose' for AM.
For the few who do enjoy WWDB, no offense intended. In fact, you get shortchanged because your analog listening of this station is compromised by the IBOC exciter and it now sounds flatter - and requires extra volume. Imagine how it would do at night. Even with an HD radio, the digital signal would fade in and out over most of the market as other stations bounce in and interfere with it. This makes for annoying listening. And, before you say "only 10 kHz on either side of the frequency is affected" that's not true for most radios. Even my best dx receivers from Sony, Sangean and Eton get hash at +/- 20 kHz from a local HD signal. So do the car radios. The average listeners don't give a crap what the guys in white coats from Ibiquity have to say about that - all they know is that the ugly interference (hash) spreads across the dial from the offending AM HD station. WOR's daytime hash is audible at +/- 10 kHz as far away as Downingtown, PA...more than 100 miles from their tx site.
In the meantime, the AM demo will grow older even faster as younger people become even more turned off by the crappy sound of the band (made worse in analog by the use of HD exciters that serve a grand total of 3 listeners).
This thing is suicide. Better to simply shut the whole band down than to do this to it. Talk about cutting off your nose to spite your face!